The present invention relates to a rotor for an electric machine. More particularly, the invention relates to an interior (or internal) permanent magnet electric machine that in one embodiment provides line-start capability.
Permanent magnet synchronous motors with line-start capability include a stator and a rotor, both of which are similar to those of an induction motor and, additionally, include at least one permanent magnet in the rotor.
Line-start permanent magnet (PM) synchronous motors, also referred to as line-fed PM synchronous electrical machines, employ a rotor winding (e.g., a squirrel cage) for torque production during motor start-up and asynchronous speed operation. At synchronous speed, the rotor permanent magnets provide field excitation and contribute to the electromagnetic synchronous torque.
Main challenges that limit the use of the line-start PM synchronous motor include the cost, which is mainly associated with the high-energy rare-earth magnets, such as NdFeB, the starting capability, especially for motors supplied from a single-phase power line and driving loads with a substantial low speed-torque, such as, for example reciprocating compressors, and manufacturing issues, such as effective means of holding the magnets in the rotor body, especially in hermetic applications.